This invention relates to ultrasonic diagnostic imaging systems and, in particular, to the use of an ultrasonic imaging systems to acquire image loops of a patient body part.
One of the advantages of diagnostic ultrasound imaging is that it enables the production of real-time images. This capability is especially advantageous in the context of echocardiography, which studies the physiology of an organ, i.e., the heart, which moves incessantly. In contrast, real-time imaging is not necessary for abdominal and obstetrical ultrasound examinations because the tissues and organs being studied are substantially stationary or very slow moving, allowing the use of static imaging.
Electrical activity of the heart generates an electrical potential on the body surface. At any given location on the body, this potential includes contributions from every region of the heart, with the contribution from a particular region being inversely proportional to the square of the distance from the region to the location on the body. Given the anatomy of the heart and chest, the potentials at most locations on the body surface represent summed electrical activity from a large region of the heart.
The body surface electrocardiogram (ECG) is a measure of electrical activity of the heart. The ECG provides a measure of the potential difference between two points on the body surface as a continuous function of time. The ECG is routinely measured using standard ECG electrodes. In ultrasound, three electrodes are commonly used to record ECG signals.
Once the electrodes have been applied to the torso, the patient""s heart can be stressed physiologically using a controlled protocol. The protocol may consist either of exercise or of pharmacological stress testing. For example, the patient may be exercised using a treadmill. Alternatives to the treadmill, such as climbing and bicycle ergometers, also may be used. In general, the stress protocol may have several stages, including control and warm-up stages, stages featuring progressively heavier stress, a relaxation stage, and a recording stage occurring between fifteen minutes and twenty-four hours after the test. Recording of ECG signals may take place during any or all of these stages.
Typically, an echocardiologist makes records of his/her ultrasound examinations for use in subsequent study and diagnosis. When an ultrasound stress examination is performed, image loops from different stress levels are acquired. An image loop is a sequence of images from one or more cardiac cycles that can be played back with endless repetition. It is important that the acquired image loops from the different stress levels be from the same projections or cut-planes of the heart, as diagnosis is based upon comparison of image loops taken from the same cross section of the heart at different stress levels.
Also in other applications of ultrasound it can be useful to compare a stored image loop with a live acquisition image in order to ensure that the newly acquired images to be compared with stored images have been acquired with the same probe position and same scan plane.
The present invention is directed to a system and a method for guiding and helping the ultrasound scanner user to acquire the correct projection/cut-plane when acquiring images of a physiological structure for comparison with previously acquired images. In accordance with the preferred embodiment of the invention, the ultrasound scanner screen area is divided in two parts, where one part is showing a reference image loop and the other part is showing the live image loop. More than one reference image loop may be shown, e.g., stress echo reference image loops both from a baseline and a previous stress level may be shown in a three-image screen layout.
The reference image loop is synchronized with the live imaging based on the patient""s ECG signal. Alternatively, some other physiological signal that varies in synchrony with the heart beat could be used, such as blood pressure, Doppler signal, a frame correlation coefficient, or even the average gray-scale from a region of interest in the image itself. The period (repetition time) of the loop is predicted using previous cardiac cycle(s).
The reference image loop in a stress examination will typically show the baseline and/or previous stress level(s), and the reference image will be automatically updated to the correct projection/cut-plane as the user is going through a protocol examination. A protocol is a predefined sequence of image acquisitions. In a stress examination the protocol defines both the projections/ cut-planes and the stress levels used in the examination in a two-dimensional matrix of images.
An essential and difficult part of ultrasound imaging is the process of finding the probe position and scan plane that gives the best possible image. When comparing images from different image acquisitions, it is important to be able to obtain the same projections (same probe position and same scan plane). The present invention makes the process easier as it provides the user with a live comparison option that can be selected during image acquisition. While acquiring images, the user can see in real time that he is aligning correctly according to the reference image loop that is used for comparison.
In a stress examination the user will be able to acquire the images faster and with better accuracy using this new technique. Using the reference image loop will make it easier to acquire the corresponding projections, thereby reducing the risk of comparing slightly different projections. If the image loops at two different stress levels are acquired at slightly different angles, there will be a risk that wall motion changes from one stress level to another will be interpreted wrongly.
Also in a stress examination, the invention gives additional information with respect to what happens to the patient during the examination, as the state of the heart at the different stress levels can be compared directly as the examination is in progress.
In accordance with the preferred embodiment of the invention, the algorithm for displaying a reference loop image in response to selection of a reference image loop display option by the system operator is implemented in software. The correct reference image loop is automatically retrieved from the image frame memory, i.e., cine memory, based on which cell in the stress protocol is active. The reference image loop is cycled with a speed that is automatically set by the software based on the current heart rate given by the live ECG acquisition. The display of the reference loop is xe2x80x9cresetxe2x80x9d (starting from the first image in the loop) at QRS trigger detection from the live EGG signal or based on a corresponding point in some other signal.
Other aspects of the invention are disclosed and claimed below.